Rubber motor mounts are used to mount motors of various kinds, but are particularly important when mounting a motor of the reciprocating piston type to the chassis of an automobile. If the mounts are made stiff enough to prevent the motor from oscillating with excessive amplitude when low frequency vibration is involved, the mounts are too stiff to adequately insulate the chassis from vibration of higher frequencies. Low frequency vibration is involved when the automobile runs on a bumpy road or the motor idles roughly, and the higher frequencies are involved when the motor runs at normal operating speeds.
The West German Pat. No. 945,899 discloses a hydraulically damped rubber motor mount having a mounting flange, a motor connection above the flange and a rubber annulus interconnecting the connection and flange and forming with them a hydraulic chamber. An elastically expansible and contractible hydraulic enclosure in the form of a rubber casing is positioned beneath the flange and is connected with the chamber above the flange via a nozzle formed by one or more orifices of substantially equal length and diameter. The chamber, enclosure and nozzle are filled with hydraulic fluid.
With this design the rubber annulus which directly carries the motor weight might be made elastically flexible enough to insulate the flange, and therefore an automobile chassis, from the higher frequencies resulting from normal motor operation. At the same time, the mount provides damping for the vibrations of low frequency because the hydraulic fluid flows back and forth between the chamber and enclosure, it being possible to design the enclosure with a resonance frequency that differs from that of the hydraulic chamber carrying the motor weight.
However, for this low frequency vibration damping there is a relationship between the damping effected and the velocity at which the hydraulic fluid can flow back and forth through the nozzle. With increasing vibration frequency the dynamic damping obtained is a corresponding increasing value because the flow resistance through the nozzle increases with the flow velocity. Unfortunately, this action continues with increasing vibration frequency until the flow resistance of the nozzle is such as to, in effect, cut off the flow with the result that the hydraulic chamber mounting the motor stiffens to a degree where it cannot absorb the higher frequency vibration. In the case of an automobile installation, this means that with the motor operating at normal speed, its vibration is transmitted through the mount to the automobile chassis.
The object of the present invention is to improve on the described German patent motor mount so as to provide a mount having equal or improved low frequency damping while at the higher frequency vibrations it provides effective insulation against their transmission through the mount to its mounting flange.